1. Field of the Invention
The present invention is directed to removing CO2 from CO2-containing gases. In particular, the present invention is directed to scrubbing CO2 from a Fischer-Tropsch GTL process gas or furnace exhaust gas using an aqueous stream, preferably at an elevated pressure, and disposing of the removed CO2 in a manner that isolates the CO2 from the environment.
2. Description of the Related Art
The conversion of remote natural gas assets into transportation fuels has become more desirable because of the need to exploit existing natural gas assets as a way to satisfy the increasing need for transportation fuels. Generally, the term xe2x80x9cremote natural gasxe2x80x9d refers to a natural gas asset that cannot be economically shipped to a commercial market by pipeline.
Conventionally, two approaches exist for converting remote natural gases into conventional transportation fuels and lubricants including, but not limited to, gasoline, diesel fuel, jet fuel, lube base stocks, and the like. The first approach comprises converting natural gas into synthesis gas by partial oxidation, followed by a Fischer-Tropsch process, and further refining resulting in Fischer-Tropsch products. The second approach comprises converting natural gas into synthesis gas by partial oxidation, followed by methanol synthesis wherein the synthesized methanol is subsequently converted into highly aromatic gasoline by a Methanol-To-Gasoline (MTG) process. Both of these approaches use synthesis gas as an intermediate. Also, while other approaches exist for using natural gas in remote locations, such approaches do not produce conventional transportation fuels and lubricants, but instead produce other petroleum products including, but not limited to, liquified natural gas (LNG) and converted methanol. The Fischer-Tropsch and MTG processes both have advantages and disadvantages. For instance, the Fischer-Tropsch process has the advantage of forming products that are highly paraffinic. Highly paraffinic products are desirable because they exhibit excellent combustion and lubricating properties. Unfortunately, a disadvantage of the Fischer-Tropsch process is that the Fischer-Tropsch process emits relatively large amounts of CO2 during the conversion of natural gas assets into saleable products. An advantage of the MTG process is that the MTG process produces highly aromatic gasoline and LPG fractions (e.g., propane and butane). However, while highly aromatic gasoline produced by the MTG process is generally suitable for use in conventional gasoline engines, highly aromatic MTG gasoline may be prone to form durene and other polymethyl aromatics having low crystallization temperatures that form solids upon standing. In addition, the MTG process is more expensive than the Fischer-Tropsch process and the products produced by the MTG process cannot be used for lubricants, diesel engine fuels or jet turbine fuels. Furthermore, like the Fischer-Tropsch process, the MTG process also generates CO2.
Catalysts and conditions for performing Fischer-Tropsch reactions are well known to those of skill in the art, and are described, for example, in EP 0 921 184A1, the contents of which are hereby incorporated by reference in their entirety. A schematic of a conventional Fischer-Tropsch process is shown in FIG. 1.
The Fischer-Tropsch process can be understood by examining the stoichiometry of the reaction that occurs during a Fischer-Tropsch process. For example, during Fischer-Tropsch processing, synthesis gas (i.e., a mixture including carbon monoxide and hydrogen), is generated, typically from at least one of three basic reactions. Typical Fischer-Tropsch reaction products include paraffins and olefins, generally represented by the formula nCH2. While this formula accurately defines mono-olefin products, it only approximately defines C5+ paraffin products. The value of n (i.e., the average carbon number of the product) is determined by reaction conditions including, but not limited to, temperature, pressure, space rate, catalyst type and synthesis gas composition. The desired net synthesis gas stoichiometry for a Fischer-Tropsch reaction is independent of the average carbon number (n) of the product and is about 2.0, as determined by the following reaction equation:
nCO+2nH2xe2x86x92nH2O+nCH2
where nCH2 represents typical Fischer-Tropsch reaction products such as, for example, olefins and paraffins. The amount of by-product water produced by the reaction is significant. For instance, when converting the molar ratios to weight ratios, one can see what the relative weight percentages of water to CH2 hydrocarbons is 56%/44%.
The three general reactions that produce synthesis gas from methane are as follows:
1. steam reforming of methane: CH4+H2Oxe2x86x92CO+3H2;
2. dry reforming, or reaction between CO2 and methane: CH4+CO2xe2x86x922CO+2H2; and
3. partial oxidation using oxygen: CH4+xc2xdO2xe2x86x92CO+2H2.
Although the above general reactions are the basic reactions used to produce synthesis gas, the ratio of hydrogen to carbon monoxide produced by the above reactions is not always adequate for the desired Fischer-Tropsch conversion ratio of 2.0. For example, in the steam reforming reaction, the resulting ratio of hydrogen to carbon monoxide is 3.0, which is higher than the desired hydrogen to carbon ratio of 2.0 for a Fischer-Tropsch conversion. Similarly, in the dry reforming reaction, the resulting hydrogen to carbon monoxide ratio is 1.0, which is lower than the desired hydrogen to carbon monoxide ratio of 2. In addition to exhibiting a hydrogen to carbon monoxide ratio that is lower than the desired ratio for a Fischer-Tropsch conversion, the above dry reforming reaction also suffers from problems associated with rapid carbon deposition. Finally, because the above partial oxidation reaction provides a hydrogen to carbon monoxide ratio of 2.0, the partial oxidation reaction is the preferred reaction for Fischer-Tropsch conversions.
In commercial practice, an amount of steam added to a partial oxidation reformer can control carbon formation. Likewise, certain amounts of CO2 can be tolerated in the feed. Thus, even though partial oxidation is the preferred reaction for Fischer-Tropsch conversions, all of the above reactions can occur, to some extent, in an oxidation reformer.
During partial oxidation, CO2 forms because the reaction is not perfectly selective. That is, some amount of methane in the reaction will react with oxygen to form CO2 by complete combustion. The reaction of methane with oxygen to form CO2 is generally represented by the following reactions:
CH4+O2xe2x86x92CO2+2H2
and
CH4+2O2xe2x86x92CO2+2H2O.
Furthermore, steam added to the reformer to control coking, or steam produced during the Fischer-Tropsch reaction can react with CO to form CO2 in a water gas shift reaction represented by the following general reaction:
CO+H2Oxe2x86x92CO2+H2.
In addition, light by-product gases, including C1-C4 hydrocarbons, are frequently used as fuel in furnaces. These fuels often include CO2 from a GTL facility along with some unreacted CO. Moreover, during operation, a furnace provides heat that can contribute to the generation of substantial amounts of CO2.
Thus, invariably a significant amount of CO2 is formed during the conversion of methane into transportation fuels and lubricants by the Fischer-Tropsch process. The CO2 produced during the Fischer-Tropsch process exits the Fischer-Tropsch/GTL process in a tail gas exiting the Fischer-Tropsch unit. Tail gases exiting a Fischer-Tropsch/GTL process comprise any gases that remain unconsumed by the Fischer-Tropsch process.
The overall proportion of carbon in methane that is converted to heavier hydrocarbon products has been estimated to be as high as about 68%. Thus, the remaining 32% can form significant amounts of CO2. These estimates of carbon efficiency are provided, for example, by Bechtel Corporation for a GTL complex using cryogenic air separation, an autothermal reformer, a slurry bed Fischer Tropsch unit and a hydrocracker for conversion of heavy wax into saleable products. See xe2x80x9cCO2 Abatement in GTL Plant: Fischer-Tropsch Synthesis,xe2x80x9d Report #PH3/15, November 2000, published by IEA Greenhouse Gas RandD Programme, the contents of which are hereby incorporated by reference in their entirety. Additionally, although the above estimates are provided for a specific GTL complex, it is believed that similar carbon efficiencies and CO2 emissions would be produced by GTL processes employing alternative technologies.
The above equations represent general stoichiometric equations, they do not reflect an optimum synthesis gas composition for the kinetics or selectivity of a Fischer-Tropsch reaction. Moreover, depending on the nature of the Fischer-Tropsch catalyst, synthesis gas ratios other than 2.0, typically less than 2.0, are used to prepare the feed to a Fischer-Tropsch unit. However, because Fischer-Tropsch units typically produce products exhibiting a hydrogen to carbon ratio of about 2.0, the limiting reagent, typically H2, is consumed first. The extra reagent, typically CO, is then recycled back to the Fischer-Tropsch unit for further conversion. Synthesis gas compositions having hydrogen to carbon ratios other than 2.0 are typically generated by recycling unused reagents.
As a result, there is an urgent need for processes that can remove CO2 from CO2-containing streams, including Fischer-Tropsch GTL process streams and furnace exhaust gases, and can dispose of the removed CO2 while effectively isolating it from the environment.
The present invention satisfies the above objectives by providing a process that removes CO2 from a CO2-containing gas by scrubbing the CO2 with an aqueous phase liquid, preferably at a pressure greater than about atmospheric pressure, and disposes of a resulting CO2-enriched aqueous stream in a manner that isolates the removed CO2 from the environment.
A process of the present invention for removing CO2 from a CO2-containing gas can include scrubbing CO2 from a CO2-containing gas using an aqueous phase liquid, forming a CO2-enriched aqueous phase. The process can further include disposing of the CO2-enriched aqueous phase in at least one of a marine environment, a terrestrial formation or combination thereof.
Also, a process, according to the present invention, for converting methane-containing gas in a Fischer-Tropsch GTL facility into liquid hydrocarbons can include converting methane-containing gas into synthesis gas. At least a portion of the synthesis gas is converted into liquid hydrocarbons and CO2-containing gas. The process further includes scrubbing CO2 from the CO2-containing gas using an aqueous phase liquid to form a CO2-enriched aqueous phase. Finally, the CO2-enriched aqueous phase is disposed of in at least one of a marine environment, a terrestrial formation or combination thereof.
Thus, in general, the present invention removes CO2 from CO2-containing gases, including Fischer-Tropsch GTL process streams and furnace exhaust gases, using an aqueous phase liquid to scrub the CO2 from the CO2-containing gas. Accordingly, one important advantage of the present invention is that it can reduce the amount of CO2 vented into the atmosphere from a Fischer-Tropsch GTL process or furnace without having to employ expensive CO2 isolation techniques including, but not limited to, gaseous CO2 compression, liquefaction or solidification.